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General Chemistry Principles, Patterns, and Applications, 2011

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onded to one hydrogen <strong>and</strong> two alkyl groups; <strong>and</strong> in tertiary amines, the nitrogen is bonded to three<br />

alkyl groups. With one lone pair of electrons <strong>and</strong> C–N bonds that are less polar than C–O bonds,<br />

ammonia <strong>and</strong> simple amines have much lower boiling points than water or alcohols with similar<br />

molecular masses. Primary amines tend to have boiling points intermediate between those of the<br />

corresponding alcohol <strong>and</strong> alkane. Moreover, secondary <strong>and</strong> tertiary amines have lower boiling points<br />

than primary amines of comparable molecular mass.<br />

Tertiary amines form cations analogous to the ammonium ion (NH4 + ), in which all four H atoms are<br />

replaced by alkyl groups. Such substances, calledquaternary ammonium salts, can be chiral if all four<br />

substituents are different. (Amines with three different substituents are also chiral because the lone pair<br />

of electrons represents a fourth substituent.)<br />

Alkylamines can be prepared by nucleophilic substitution reactions of alkyl halides with ammonia or<br />

other amines:<br />

Equation 24.12<br />

RCl + NH 3 → RNH 2 + HCl<br />

Equation 24.13<br />

RCl + R′NH 2 → RR′NH + HCl<br />

Equation 24.14<br />

RCl + R′R″NH → RR′R″N + HCl<br />

The primary amine formed in the first reaction (Equation 24.12) can react with more alkyl halide to<br />

generate a secondary amine (Equation 24.13), which in turn can react to form a tertiary amine (Equation<br />

24.14). Consequently, the actual reaction mixture contains primary, secondary, <strong>and</strong> tertiary amines <strong>and</strong><br />

even quaternary ammonium salts.<br />

The reactions of amines are dominated by two properties: their ability to act as weak bases <strong>and</strong> their<br />

tendency to act as nucleophiles, both of which are due to the presence of the lone pair of electrons on the<br />

nitrogen atom. Amines typically behave as bases by accepting a proton from an acid to form an<br />

ammonium salt, as in the reaction of triethylamine (the ethyl group is represented as Et) with aqueous<br />

HCl (the lone pair of electrons on nitrogen is shown):<br />

Equation 24.15<br />

Et 3 N:(l) + HCl(aq) → Et 3 NH + Cl − (aq)<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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